Debit cards allow holders to quickly pay for goods and services using money held in bank accounts. Normally, debit transactions come from money that a cardholder has stored in his checking account. However, there are some cases in which debit cards tap into money in a cardholder's savings account.
When a cardholder uses a debit card to withdraw cash from an ATM, he normally has a choice whether to pull that money from his checking account or from his savings account.
Some banks allow a cardholder to link his savings account to his checking account in case of an overdraft. When a cardholder accidentally overdraws his checking account when making a debit card purchase, money can be drawn from his savings account to pay the difference.